98 CHEMICAL ENGINEERING A simple equation for ascertaining the relative cost of common sizes of screw conveyors is given as Formula 17, but the one for calculating the relative depreciation per year is complicated by the introduction of the depreciation factor necessarily entailed when the moving parts of the conveyor are in direct contact with the load in a manner which produces a destructive abrasive, or scouring, action. RC8C = 0.196dL -f 2.333d + 0.07d2 Formula (17) Where, RC8e = Relative cost of screw conveyor, d — Diameter of screw in inches, and L = Length of conveyor in feet. RD8C = (O.OGldL -f- 0.25rf -f- 0.007d2)^ Formula (18) Where, RDSC = Relative depreciation (annual) of screw conveyor, and F = Depreciation factor =0.47 for coke 1.00 for fine coal 0 . 99 for ashes 1 . 15 for lime and cement " Fixed charges for screw conveyors may be taken as 8.5 per cent a year by allowing an annual renewal charge of 20 per cent of initial cost. Labor expense of about % ct. per inch diameter of screw during hours of operation and a supply expense of 1% cts. per horsepower constitute the other charges included in the net cost of operation. The total cost of operating a screw conveyor is best indicated by the consideration of a typical example, as follows: Example. — Required the net operating cost per ton of a horizontal screw conveyor, 12-in. diameter and 24 ft. long, handling 10,000 tons of cement — weighing 65 Ib. per cubic foot — over a period of a year. Power delivered at conveyor drive commanding a value of 3 cts. per horsepower. Advisable speed of conveyor (Table 10) 150 r.p.m. Capacity, W = °-3213 365 x 15° = 12.81— say 10 tons per hour. Formula (15) Required operating hours (10,000 •*- 10) . ........................ 1,000 hr. Power requirements: Horsepower » °'68 *0faX ^ = 0.1632— say 14 horsepower. Formula (16) Costs : RC8C= 0.196 X 12 X 24 + 2.333 X 12 + 0.07 X T22_= 94.424— say 96. Formula (17) RDSC = (0.061 X 12 X 24 + 0.25 X 12 -f 0.007 X I22)1.15 = 24.81— say 26. Formula (18) Take market price at 25 per cent above relative costs. Cost = 1.25 X 96 = $120.00 .......................... initial cost of equipment. Fixed charges: (0.085 X 120)........................... $ 10.20 Labor charge: (0.0075 X 12 X 1,000)................... 90.00 Supply expense: (0.01125 X 0.25 X 1,000)................ 2.80 Power charge: (0.03 X 0.25 X 1,000)................... 7.50 Depreciation: (1.25 X 26)............................. 32.50 Renewal charge: (0.20 X 120)............................ 24.00 $167.00 Net operating cost: (167 -5- 10,000).................................. $0.0167 per ton.